Photophysical Studies of Ring Containing Conjugated Polymers

Ultrafast and cw spectroscopy of ring-containing conjugated polymers, in particular, leucoemeraldine (LEB), emeraldine (EB) and pernigraniline bases (PNB) of polyaniline, and poly(p-pyridine)(PPy), are investigated. The important roles of ring-torsional degrees of freedom were demonstrated by investigating the dynamics of photoexcitations in restrained (solid) and unrestrained (solution) environments. When excited into the 2.0 eV exciton band (EB) or the 2.3 eV Peierls gap (PNB), the photoexcited species in NMP solution decayed within 30 ps. However, under similar experimental conditions, a long-lived component (>ns) appeared in solid samples of EB and PNB. For EB, the 2.9 eV polaron (rm P_{B}^ {+}) lasted longer than the exciton ( tau_{rm polaron}>tau _{rm exciton}). The situation was reversed for PNB, in which the polarons decayed faster than the recovery of the ground state. Comparing the PIA results, the lifetime of the polaron follows: tau_{polaron} (LEB)> tau _{polaron} (EB)>tau_{polaron }(PNB). These results strongly suggest that the quinoid moiety in polyaniline is acting as a quenching site for P_{B}^ {+}. Intensity dependence studies have revealed that the decay mechanism is mainly unimolecular for both the 2.0 eV band (PIB) and the 2.9 eV band (PIA) of EB and PNB. It appears that in polyaniline the confinement length is longer than the decay length of the 2. 1 eV photoexcitation. Photoluminescence (PL) studies of PPy have confirmed the notion that optically active species in conjugated polymers are comprised of sub-units of the chain likely separated by topological defects. The PL of PPy features a minimum Stokes shift when chains of the polymer are unrestrained, such as in solution. Whereas, a persistent apparent Stokes shift is present for the PL occurring in a confined environment, such as films. Phosphorescence and fluorescence of rm (n-pi*)^1 state were detected at photoexcitation energies significantly above the band edge for PPy/ITO/glass films. Time-resolved fluorescence studies revealed red shifting of the PL with the evolution of time, which is a signature of spectral diffusion and is further proof for the statistical nature of the chain segments. We suggest the nature of photoexcitations in PPy is primarily excitonic.